The Australian father of two had recently returned to New Zealand and was temporarily living with his ex-partner at Taieri Mouth on the morning of February 21.
The pair argued about Roker's alleged drug use and she started to move his belongings outside.
Defence counsel Anne Stevens said her client had been tested immediately after the incident and had no illegal drugs in his system.
Roker struck his former partner in the head with his elbow and then hit her with an open hand, bruising her face, the court heard.
Two hours later, her father came home, saw her injuries and confronted the defendant, telling him to leave the property
Roker began "ranting" about who was at fault then attacked his ex-partner's father.
He managed to block an initial blow and the pair tussled on the floor of the lounge, with the victim gaining the upper hand.
Roker ran from the house to where his belongings had been piled, at the bottom of some steps.
"He removed a large machete from his belongings, raised it above his head and ran at both victims who had followed him down the landing," a summary said.
Roker aimed his blow at the man's head but he raised his left arm to block it.
"The machete sliced through the victim's arm, severing it just above the left wrist," the court heard.
The woman shielded her father from further injury and took him inside to the bathroom.
She tried to hold a door closed to prevent Roker gaining access but he smashed a glass panel with the machete.
However, when he reached the bathroom and saw the damage he had caused, the defendant "calmed down" and provided first aid, which included applying a tourniquet to stem the blood flow.
The victim required extensive reconstructive surgery, Justice Gendall said, but the "long-term efficacy remains unclear".
Mrs Stevens said the background to the incident was key.
Roker had only left Australia to rejoin his ex-partner at her request.
"Unfortunately he arrived to a somewhat volatile situation," she said.
Mrs Stevens described him as someone of "good character" and the court heard that though the defendant had previous convictions, they were driving offences rather than violence.
Roker's mother had come over from Australia for today's sentencing. His father had died of cancer while he was on remand.
Justice Gendall ordered Roker pay $2000 to the victims for the emotional harm caused.
A minimum non-parole period, requested by the Crown, was declined by the judge.
